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Upper Permian Reef and Shelf Facies of the Southern Margin of the Yangtze Platform, South China Provide Insight Into Reef Ecology and Environments Prior to the End-Permian Mass Extinction

Abstract

The Nanpanjiang Basin of south China is an embayment in the southern margin of the Yangtze microcontinent that was located in the tropical eastern Tethys during Permian-Triassic time. The basin is bordered to the north by the Yangtze Platform, a vast shallow-marine carbonate shelf. Upper Permian reef facies along the southern margin of the Yangtze Platform are massive boundstone with a primary framework of inozoan and sphinctozoan sponges bound by laminated microbial crusts, Tubiphytes and marine cements. Secondary frameworks are constructed by rugose corals, branching bryozoans and Tubiphytes. Organisms dwelling within the framework include fusulinids, smaller foraminifers, brachiopods, bivalves, and dasycladacean algae. We found no evidence of bioerosion. Backreef strata consist of finely comminuted tubular microproblematica and peloidal grainstone, mud-rich skeletal packstone with fragments of large open-marine organisms, and minor sponge-coral boundstone. The fine comminuted grainstone represents the shallowest subtidal high-energy environments, whereas the packstone and boundstone represent deeper subtidal low energy environments. Facies patterns in the overlying basal Triassic microbialite facies parallel trends in the uppermost Permian: Thinner microbialite development represents lesser creation of accommodation space over shallower, higher energy areas in the Upper Permian, thicker microbialite represents greater accommodation developed over the deeper low energy areas of the Upper Permian. Basal Triassic microbialites contain a greater proportion of skeletal grainstone interbeds indicating greater current energy on the ramp facing the open ocean to the south. The Upper Permian backreef facies lack this trend, likely reflecting protection from hydrodynamic energy of the open ocean by a barrier reef. Despite the dramatic biotic turnover associated with the end-Permian extinction, the Upper Permian and Middle Triassic reefs of south China are very similar. Sponges are the dominant metazoan framebuilder in both. Both contain frameworks of tubular micrite walled fossils and are heavily bound by microbial and marine cement crusts. Even though these boundstones have a significant metazoan framework component, they also have a large (in some cases dominant) volume of microbial and abiotic (cement) precipitates. Seawater chemistry (anoxia and carbonate saturation) rather than evolutionary trends likely explains the large microbial and abiotic component.